Colder nights are closing in and thermostats are edging up. Across the UK, families face hard choices about warmth and money.
Into that mix steps consumer champion Martin Lewis, reviving a familiar seasonal debate: should you leave your heating on low all day, or only switch it on when you need it? His guidance remains clear, but the details—and your home—matter more than many think.
What Martin Lewis actually advises
Lewis has long said most households spend less by warming the home only when required. Let the thermostat and a simple timer do the heavy lifting. Heat to a comfortable temperature when you are at home and active. Let the system rest when you are out or asleep.
Use heat when you need it. A timer and thermostat control costs better than constant low-level heating.
He also flags an exception. Properties that trap moisture—or suffer chronic condensation—can sometimes lose heat faster after long “off” periods. In these homes, a gentle, steady background temperature may cut damp and reduce overall heat loss.
Damp-prone homes can behave differently: moisture in walls conducts heat away, so a low, steady background may help.
Why the ‘on all day’ myth persists
The appeal is obvious: steady warmth feels comfortable and avoids big temperature swings. But physics rarely agrees. Heat constantly flows from warm air and surfaces to colder outdoors. Keeping the whole house warmer for longer extends the time you are losing energy through walls, windows and roofs.
Two factors muddy the picture. First, insulation quality. A modern, well-sealed home retains heat so well that short, sharp heating periods often suffice. Second, moisture. If your walls are damp, they can carry heat away more quickly after the system switches off, making recovery cycles costlier.
The moisture question
Condensation forms when warm, moist air hits cold surfaces. Switch the heating off, surfaces cool, and vapour condenses into walls and corners. That water conducts heat better than dry air and can increase heat loss when you warm the place again. Ventilation, dehumidification and repairs to leaks or cold bridges should come first. If problems persist, a low background setting in problem rooms can stabilise humidity and temperature.
Numbers that matter: rough costs at the meter
Energy bills reflect how many kilowatt-hours you actually use. Constant heating often means more hours near your thermostat setpoint, so more kWh. Timed heating usually means fewer hours near setpoint, so fewer kWh.
A simple illustration helps. Suppose gas costs 7p per kWh and your system uses roughly 8 kWh to keep a modest home at 20C for an hour on a cold evening. Four hours of evening heat costs about £2.24. Holding a background 16C all day might draw 3 kWh each hour for 24 hours in a leaky home—£5.04. That is more than double. Your figures will differ, but the principle stands: time at temperature drives cost.
| Approach | Pros | Cons | Best for |
|---|---|---|---|
| On when needed (timer + thermostat) | Lower kWh, flexible comfort, easy to adjust | Cooler start-up, needs planning | Most gas-boiler homes, decent insulation |
| Low all day | Stable temperature, fewer cold spots | More hours losing heat, higher kWh | Damp-prone homes while fixing moisture |
| “Set and forget” smart schedules | Adapts to habits, geofencing, weather tweaks | Upfront cost, setup time | Busy households with variable routines |
How to set your home up for cheaper warmth
- Use a timer: warm rooms before you wake and return, then let the system rest.
- Pick a target: 18–21C suits most living spaces; keep bedrooms cooler for sleep.
- Fit and use TRVs: set lower in spare rooms; keep doors closed to avoid wasting heat.
- Lower the boiler flow temperature: condensing gas boilers run more efficiently around 50–60C to the radiators.
- Bleed and balance radiators: even heat across the system saves energy.
- Seal draughts: letterbox brushes, chimney balloons, and tidy gap sealing reduce heat loss.
- Tackle moisture: vent kitchens and bathrooms, dry clothes outside of living spaces, consider a quiet dehumidifier.
- Protect pipes: if away in freezing weather, keep the heating at 7–10C to prevent bursts.
What real households report
In discussions on money forums, one common pattern emerges: people set a comfortable temperature when at home and drop it to around 10C when out. Another frequent approach in new builds is a brief morning warm-up and a longer early evening period, with the system off for the rest of the day. Both strategies rely on timers and thermostats, and both lean on good insulation to hold heat between cycles.
When leaving it on low can make sense
Some homes are stubbornly damp, poorly insulated, or listed, making upgrades difficult. If switching the system off leads to dripping windows, musty smells, and cold-soaked walls, a gentle background setting in key rooms can stabilise conditions while you improve ventilation and repair defects. People with health needs who cannot tolerate cool periods may also benefit from steadier warmth.
If condensation bites and upgrades are limited, a low, steady background in problem rooms can cut discomfort and heat loss peaks.
Heat pumps and electric systems
If you heat with an air-source or ground-source heat pump, the picture shifts. These systems deliver low-temperature heat efficiently over long periods. In many pump-heated homes, “low and slow” with weather compensation works better than hard daily peaks. Night-time setbacks still help, but large temperature drops can backfire by forcing the pump to work harder. Storage heaters tell a different story again: charge overnight on off‑peak, release through the day, and top up sparingly on peak-rate electricity.
A quick home test you can run next week
Two simple trials will reveal what your building really does:
- Week A: use a timer with two heating windows (for example, 6–8am and 5–9pm). Record daily meter kWh and average thermostat settings.
- Week B: try a low, steady background (for example, 16C all day) with small evening boosts. Record the same data.
Compare kWh totals, not just cost, as tariffs vary. Pick the pattern that uses fewer kWh while keeping you comfortable. Repeat during a colder spell to confirm the result.
Measure, do not guess: the cheapest schedule in your home is the one that uses the fewest kWh for acceptable comfort.
Extra ways to squeeze bills down this winter
Small tweaks stack up. Swap heavy curtains onto the coldest windows and close them at dusk. Move furniture off radiators to let heat circulate. Lay a simple door snake on draughty thresholds. If you have a hot-water cylinder, set it to the recommended temperature and insulate it well. Consider upgrading attic insulation and adding secondary glazing film on the worst windows. These changes cost little compared with running your boiler for extra hours.
For those able to invest, smart thermostats with occupancy sensing, weather compensation controls for boilers, and basic fabric upgrades can reduce daily runtime without sacrificing comfort. If you rent, ask for permission to fit draught-proofing tape and TRVs, and keep an eye on humidity to prevent damage from mould.
For a damp Victorian terrace, would a steady 16C background with small evening boosts actually cost less than strict on-when-needed? Anyone run the Week A vs Week B meter test and compare kwh, not just £? Real numbers please.